Foray into Solar, Continued -- What "45 Watts" Really Means When It Applies to Solar Panels

In the previous post, I discussed how the kit I bought did not make it clear that a battery is needed to make the system function properly.  Without a battery, you end up with flickering lights and appliances that don't run properly.  Another thing the kit does not make clear is that you're not going to get the advertised power.

The manufacturer calls my system a "45 Watt Solar Panel Kit", but the system actually produces significantly less power. This post explains how this can be the case.

My system is configured as illustrated here:

The three solar panels are connected in parallel through a junction, feeding their power to the charge controller.  The charge controller smooths out the incoming power, and applies it to the battery to charge it.

The charge controller also dispenses power from the battery to run lights and other appliances.

During operation, the solar panels erratically produce 13-18 volts and a maximum current of about 2 amps. This means that the maximum total power coming from the solar panels (assuming the maximum volts and amps occur simultaneously) is about 18 volts x 2 amps, which is 36 watts.  But the kit claims that the solar panels produce 45 watts.  "Watts" the deal?

It turns out that the way the panels apparently are rated is by multiplying the open circuit voltage (i.e., the voltage across the lines coming from the junction of the panels when the lines are disconnected from the charge controller) and the short circuit current (the current from the solar panels when the lines from the junction are shorted).  The open circuit voltage is the maximum voltage you can get from the panels, while the short circuit current is the maximum current you can get.  These are two separate extreme cases, which means you don't ever get this maximum voltage and maximum current at the same time.

The open circuit voltage turns out to be about 20 volts, while the short circuit current is about 2.2 amps.  Multiplying these two values you get 44 watts, which is fairly close to 45 watts.  I find this rating approach deceptive, because you'll never actually get 45 or even 44 watts.

What you'll actually get becomes more clear when you look at the graph of volts vs. amps for the solar panels:

The blue curve shows the relationship between current and voltage for the solar panels.  The current starts at 2.2 amps at the short circuit state (0 volts), and reaches zero at the open circuit state (20 volts).

The orange curve shows actual power output.  The maximum power point of 36 watts is somewhere around 18 volts where the current is 2 amps (18 volts x 2 amps = 36 watts).  This is markedly lower than the advertised power output of 45 watts.

The important lesson here is when considering solar panels, be sure you know what the power rating is referring to -- the actual power you'll get, or a fictitious level that combines the open circuit voltage and the short circuit current.  This is especially important if you're putting solar panels on your roof or filling a field with them.  It's one thing to find out that your "45 watt" kit produces only 36 watts.  It's quite another thing if your 7500 watt system turns out to produce significantly less.

Next I'll address if my system would be a practical power source during a power outage, and if not, what would be needed?